Internal oxidized Ag-Sn system alloys are widely used today as electrical contact materials with excellent physical and electrical properties. They are made by internally oxidizing Ag alloys which have been molten, cast, and rolled or drawn, and are different from those sintered Ag-metal oxides alloys which are made by mixing matric Ag powders with powders of the metal oxides and sintering them. One of their noticeable differences is that the former, viz. internal oxidized Ag-Sn system alloys are far superior to the latter in respect to structural density, while the latter has the more uniform dispersion of metal oxides than the former. The latter may be very readily consumed in too rapid and frequent switching operations.
Therefore, it is a problem to be solved for internal oxidized Ag-Sn system alloys how to make tin oxides dispersed uniformly throughout their internal oxidized Ag matrices. Compared to internal oxidized An-Sn system alloys, internal oxidized Ag-Cd system alloys which had competed with the Ag-Sn system alloys, have a more uniform dispersion of metal oxides. This is chiefly because that the diffusion velocity of Cd in a silver matrix is inherently well balanced with the diffusion velocity of oxygen in the internal oxidation, while they are not so in the case of internal oxidation of Ag-Sn system alloys. In other words, electrical contact materials made of internal oxidized Ag-Cd system alloy and methods for preparing them can hardly be references which are utilizable for the preparation of Ag-Sn system alloys and the internal oxidation thereof.
When Ag-Sn system alloys are subjected to the complete internal oxidation even with the addition of auxiliary solute elements such as In and Bi, it is unavoidable to see segregation of Sn oxides at their outer areas and depletion of said oxides at their inner areas. The segregation of tin oxides which takes sometimes the form of crust about contact surfaces, is chiefly due to an unbalance between diffusion velocities of Sn and oxygen as discussed above. Besides, the diffusion velocity of oxygen itself about the contact surfaces is subject to sharp fluctuations on account of scabs which have been produced about the contact surfaces when alloys were rolled to sheets or drawn to wires. Since oxygen combined to tin in the internal oxidation is double compared to the oxidation of Cd, the aforementioned kind of fluctuations of diffusion velocity of oxygen is serious.
At all events, the segregation of tin oxides about contact surfaces makes them too hard, and often brings about cracks of the surfaces. High electrical contact resistances especially of an initial stage of operations of electrical contacts made from internal oxidized Ag-Sn alloys result from the segregation or excessive concentration of tin oxides about top surfaces. Unduly high raise of temperature of contacts results also from the segregation.